1. Field of the Invention
The present invention relates to a process for manufacturing polyarylene sulfide, and, more particularly, to a process for manufacturing high quality polyarylene sulfide, useful as a material for electrical or electronic parts and appliances and as a high rigidity material, at a low cost.
The present invention further relates to a process for separating a solid compound other than lithium hydroxide (such a solid compound is hereinafter referred to as non-lithium hydroxide solid compound), and, in particular, to a process for efficiently separating a non-lithium hydroxide solid compound produced in the process for manufacturing polyarylene sulfide.
Furthermore, the present invention relates to a process for recovering lithium ion, and, in particular, to a process for efficiently recovering lithium ion from a reaction mixture of polyarylene sulfide containing lithium chloride in the process for manufacturing polyarylene sulfide.
2 . Description of the Background Art
Polyarylene sulfide resins (PAS resins), in particular, polyphenylene sulfide resins (PPS resins), are known as engineering plastics having excellent mechanical strength, heat resistance, and especially high rigidity. They are useful materials for electrical and electronic parts and appliances, and as high rigidity materials for various applications. A conventional process for manufacturing these resins is reacting a dihalogeno aromatic compound such as p-dichlorobenzene and a sodium salt such as sodium sulfide in a non-protonic organic solvent such as N-methyl-2-pyrrolidone (hereinafter abbreviated from time to time as MMP). A problem in this process resides in the difficulty in removing by-produced sodium chloride from resins by washing, since sodium chloride is insoluble in the solvents such as NMP and is incorporated in the resins. A process using a lithium salt instead of the sodium salt is attracting attention as a process for overcoming this problem. Because lithium chloride produced as a side product in the polymerization reaction is soluble in many of the non-protonic organic solvents such as NMP, it is possible to comparatively easily reduce the lithium content in the resin.
In order to reduce the manufacturing cost of the resins, however, it is essential to recover and reuse lithium which is far more expensive than sodium.
As a process for manufacturing polyarylene sulfide resins, including polyphenylene sulfide resins, using the lithium salt, U.S. Pat. No. 4,451,643 discloses a batch or continuous process for manufacturing these resins by reacting lithium N-methylaminobutyrate (hereinafter abbreviated from time to time as LMAB), produced by the reaction of lithium hydroxide and NMP, a dihalogeno aromatic compound, such as p-dichlorobenzene, and hydrogen sulfide. In this process, polymers such as PPS resins are synthesized using LMAB, which is a lithium salt but not a sodium salt, as a polymerization raw material, whereby lithium chloride which is soluble in NMP solvent is produced as a side product. Because of this, the amount of alkali metal components incorporated in the polymers is greatly reduced as compared with the conventional process wherein sodium chloride which is insoluble in the solvent is produced as a side product. Nevertheless, this process has a drawback in that LMBA must be first produced by the reaction of lithium hydroxide (LiOH.multidot.H.sub.2 O) and NMP.
Moreover, because this process uses lithium hydroxide as a starting raw material, lithium chloride produced as a side product by the polymerization reaction must be regenerated into lithium hydroxide for recovering and reuse of this compound. As a method for collecting lithium chloride and converting it into lithium hydroxide, U.S. Pat. No. 4,451,643 proposes a method of washing the reaction products and the polymers with water to collect lithium chloride as an aqueous solution, and reacting the lithium chloride with sodium hydrogen carbonate or subjecting the aqueous solution to electrolysis to convert the lithium chloride into lithium hydroxide. The method of using sodium hydrogen carbonate requires a complicated process wherein lithium chloride is first converted to lithium carbonate and then to lithium hydroxide. The electrolysis is disadvantageous in view of production cost and is not industrially viable.
The process proposed by U.S. Pat. No. 4,451,643 requires a step for producing lithium N-methyl-aminobutyrate from lithium hydroxide and N-methyl-2-pyrrolidone, wherein a large quantity of water is required for preventing a retarded reaction which takes place when the amount of water is small. This is another drawback which makes the process industrially less attractive.
Japanese Patent Application Laid-open (kokai) No. 180928/1990 discloses a process for manufacturing high polymeric PAS, wherein a mixture of an organic polar solvent, sodium hydroxide hydrate, and alkali metal aminocarboxylate is dehydrated by distillation or the like, and the dehydrated mixture is mixed and reacted with lithium halogenide and a dihalogeno aromatic compound. This process was proposed in order to overcome the difficulty in removing crystal water contained in alkali metal sulfide which was used in a process for manufacturing PAS.
However, the mixture to be dehydrated in this process is a slurry which involves a problem when it is subjected to continuous distillation while heating. Although there are no problems when the slurry is treated by batch distillation, in the continuous distillation, the mixture is continuously sent to a distillation column whereby components are separated by boiling point differences among them, removing vapor from the top. Solid components in the slurry may stay and accumulate in column trays, choke up the trays, and bring the distillation column inoperative.
In a process for separating a non-lithium hydroxide solid compound such as alkali metal chloride from a mixture containing lithium hydroxide and the non-lithium hydroxide solid compound in N-methylpyrrolidone, a method of feeding water to the system to separate the alkali metal chloride from lithium hydroxide by the utilization of difference in the solubility in water between the two components has been commonly adopted. However, the separation efficiency of this method was extremely poor, because the difference in the solubility in water between lithium hydroxide and alkali metal chloride is small. Another method which has been adopted is feeding water to the system and heating the mixture to convert the lithium hydroxide into lithium N-methylaminobutyrate which is soluble in N-methylpyrrolidone. Alkali metal chloride existing as a solid is then separated. This method has problems in that the procedure is complicated and a large quantity of water is left in the system. Further, lithium hydroxide cannot be separated efficiently if the mixture contains polyarylene sulfide oligomers and the like in addition to alkali metal chloride.
The present invention has been achieved in order to solve the above problems, and has an object of providing an efficient and simple process for manufacturing polyarylene sulfide containing only a slight amount of alkali metal chloride and the like as impurities and having high quality, whereby recover and reuse of lithium is possible.
Another object of the present invention is to provide a process for manufacturing high polymeric and high quality polyarylene sulfide, wherein the reaction mixture can be dehydrated by continuous distillation while preventing the distillation column trays from being choked up.
Still another object of the present invention is to provide a process for efficiently separating a non-lithium hydroxide solid compound such as alkali metal chloride from a mixture containing lithium hydroxide and the non-lithium hydroxide solid compound in a non-protonic organic solvent.
A further object of the present invention is to provide a process for recovering lithium ion from lithium chloride produced in a process for manufacturing polyarylene sulfide.